The invention relates to receptors, particularly adenosine receptors.
Adenosine modulates a variety of physiological functions by acting through specific cell surface receptors (Williams, 1987, Annu. Rev. Pharmacol Toxicol, 27:315-345; Linden, 1991, FASEB J, 5:2668-2676). These receptors are coupled to guanine nucleotide binding proteins (G proteins) and have been broadly divided into A.sub.1 - and A.sub.2 -receptor subtypes. Each subtype has a specific pattern of ligand binding, a unique tissue distribution, and a distinct action on the cAMP regulatory system; A.sub.1 -adenosine receptors inhibit, while A.sub.2 -adenosine receptors stimulate, adenylyl cyclase activity (Van Calker et al., 1979, J. Neurochem., 33:999-1005; Londos et al., 1980, Proc. Natl. Acad. Sci. USA 77:2551-2554).
Using the polymerase chain reaction (PCR), high affinity A.sub.1 - and A.sub.2 a-adenosine receptor cDNAs have been recently cloned from dog thyroid gland (Libert et al., 1989, Science 244:569-572; Maenhaut et al., 1990, Biochem. Biophys. Res. Commun. 173:1169-1178; Libert et al., 1991, EMBO J. 10:1677-1682) and rat brain (Reppert et al., 1991, Mol. Endocrinol. 5:1037-1048; Mahan et al., 1991, Mol. Pharmacol. 40:1-7). The A.sub.1 - and A.sub.2a -adenosine receptor cDNAs are structurally similar to each other but have features which clearly distinguish them from monoamine and peptide receptors. Thus, adenosine receptors appear to comprise a new subfamily within the rapidly growing superfamily of G protein-coupled receptors.